Lohse and colleagues first prepared a sand bed, around 25 cm thick, from fine sand grains measuring on average 50 microns across. The sand was "decompactified" by blowing air through it and then allowed to settle in an extremely loose-packed structure, so that it essentially behaved like a fluid. Next, the scientists dropped a steel ball, with a diameter of 2.5 cm, onto the sand from various heights and angles while taking images with a high-speed digital camera.

The Twente team observed a series of well-defined steps: on impact, sand is blown away in all directions to form a crown-shaped splash. The ball then penetrates the sand and creates a void, which then collapses under the influence of the hydrostatic-like pressure of the sand. This pressure subsequently ejects sand grains into the air to form jets (see figure). Using numerical simulations the scientists developed a theory to explain how the void collapsed.

"We have shown that the impact of an object on loosely packed granular material can be well described by a simple, fluid dynamical continuum model. So in our system sand behaves like water!" team member Devaraj van der Meer told PhysicsWeb. "This is very surprising since it has often been argued that, in general, no continuum description of granular materials is possible," he added.

"There is a striking similarity with the large-scale impact of meteors and other celestial objects on the surface of the Earth -- for example the Chixulub impact crater in Yucatan, Mexico, thought to be responsible for the extinction of the dinosaurs -- and our experiment," said van der Meer. "Our scaled-down granular experiments under laboratory conditions possibly capture the essential features of these crucial events in the history of our planet."